Alumina trihydrate (also known as aluminum hydroxide, alumina, and gibbsite) is produced on an industrial scale in the Bayer process. As shown in FIG. 8, in this process, a raw ore 1 commonly known as bauxite is contacted with hot caustic soda solution in the digesters 3. This results in the dissolution (digestion) of a considerable portion of the aluminum-bearing minerals, affording a supersaturated solution of sodium aluminate (pregnant liquor) which is flash cooled in flash coolers 5. The cooled pregnant liquor is physically separated from the undigested minerals and other solids in settling tanks 7 and filter presses 9. After the solids are removed from the pregnant liquor, the liquor is again cooled in a flash cooler 13. After the physical separation of undigested mineral residues (red mud) 11, the sodium aluminate solution is precipitated 15 to afford alumina trihydrate, which is recovered by filtration 17 and calcined 19 in a kiln. This precipitation step is promoted by the addition of fine alumina trihydrate seed crystals. The remaining liquor is referred to as depleted or "spent liquor". The depleted or spent liquor is free of suspended solids. After the precipitation step, the spent liquor is then recycled extensively, accumulating a variety of dissolved inorganic and organic species over time. During the recycle loop of the process, the spent liquor is heated by heaters 21 and concentrated in evaporator 23. The spent liquor is recycled into the digestion tanks.
The bauxite ores used in this process are found in many parts of the world, and the composition of the ore varies from one location to the next. Generally, bauxites are composed of mixtures of inorganic minerals including oxides and hydroxides of the elements aluminum, iron, titanium and silicon, silicates and aluminosilicates (clays), and organic matter.
During the digestion of bauxite ore, the attack of caustic soda on certain silica-bearing components in the ore results in the release of soluble silicate species into the liquor. These soluble silicates then react with alumina and soda to form insoluble sodium aluminosilicates which are also known as desilication products, or DSP.
Silica present in bauxite as clays is particularly susceptible to dissolution by caustic attack. Silica in this form can constitute as much as 5% of the total mass of the bauxite. Silica present as quartz is more resistant to caustic attack and dissolution, and can constitute as much as 10% of the total mass of the bauxite.
The chemical composition of desilication products can vary from one plant to the next owing to differences in plant operating conditions and liquor chemistry. Furthermore, desilication product compositions can vary within a particular plant, depending on the processing temperature and chemical composition at any given point. Nevertheless, many of the desilication products described in the literature conform approximately to the general formula for the sodalite family of minerals:
Na.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2.1/3(Na.sub.2, Ca)(2Cl, SO.sub.4, CO.sub.3, S, 2OH, 2AlO.sub.2, etc.)
Sodalite itself is the chloride mineral, whereas the sulfate and carbonate forms are known as noselite and cancrinite, respectively. Quite often, desilication product samples are found to be physical mixtures of several of these compounds. Deviations from the stoichiometry of the general sodalite formula are commonly observed. For example, deficiencies in the amount of soda compared with that expected for a true sodalite have been attributed to the replacement of sodium by hydrogen from wash water.
Much of the desilication product exits the plant in the red mud, but a significant portion of soluble silica remains in the pregnant liquor. A small but significant amount of this silica appears as a contaminant in the alumina trihydrate. Furthermore, desilication product is deposited as scale on the walls of pipes and vessels in the recycle loop of the plant which includes heaters 21 and evaporators 23. Scaling by DSP is particularly severe on heated equipment surfaces, such as heat exchanger tubes in heaters 21. The invention encompasses a method of reducing the formation of insoluble siliceous deposits upon Bayer process equipment during the thermal treatment of spent Bayer process liquor.
The presence of large quantities of soluble siliceous species in spent Bayer liquors and their subsequent transformation into insoluble desilication products is detrimental to the operation of the Bayer process, and therefore, undesirable for several reasons. From the general sodalite formula, it is evident that potentially saleable alumina and costly soda are diverted into desilication products. As the recovery of the alumina and soda is not economically viable, these diversions constitute substantial process losses which are economically significant on an industrial scale. Furthermore, the deposition of desilication products on process equipment surfaces such as on the walls of heat exchanger tubes and pipes reduce their operating efficiencies. For example, the flow of fluids through pipes can be impeded by the accumulation of desilication products as scale on the pipe walls, In heat exchangers, scaling of the tubing walls by desilication products can seriously impede the transfer of heat from the steam side which furnishes heat, to the process liquor which is being heated. The removal of such scales is accomplished by manual and/or chemical means, both of which constitute additional costs to the plant.
The negative impact of desilication products on alumina quality and on Bayer Process efficiency and economics has prompted alumina producers to devise methods for mitigating these detrimental effects.
Much effort has been directed at developing processing methods, which are practiced in conjunction with the digestion of bauxite and which promote the dissolution of the siliceous constituents of bauxite and the subsequent precipitation of desilication products to the greatest extent possible. By depleting the liquor of silica at this stage, the potential for scaling downstream is thereby reduced. For example, the temperature or time of digestion may be increased to promote the desilication reactions. In addition, low-silica bauxites may be subjected to a predigestion desilication step 25 in which the concentrated spent liquor slurry of ground bauxite is held at close to atmospheric boiling temperature for periods of 8-24 hours. By this method, fine, high-surface area desilication product seed is formed. When combined with the main digestion slurry, this seed is very effective in promoting the crystallization of desilication product. To raise the concentration of silica above the supersaturation level beyond which desilication products will form, it may also be necessary to add a clay or other siliceous materials in the digestion or predigestion desilication steps. These methods have been summarized in articles such as "Control of Silica in the Bayer Process Used For Alumina Production," S. Ostap, Canadian Metallurgical Quarterly, Vol. 25, No. 2, pp. 101-106.
In addition, a wide variety of inorganic salts and compounds, such as oxides, hydroxides, silicates, aluminates, and other forms of calcium, magnesium, and barium have been employed to control desilication of Bayer liquors.
The use of organic treatments to control deposition of desilication products on equipment surfaces is a relatively unexplored area. A hydrophobic organosilicon liquid polymer has been reported to inhibit the formation of scales during heating and evaporation of aluminate liquors (Kazakov, V. G., Potapov; N. G.; Bobroy, A. E., Tsvetn. Met,, 1979, (1), 45-48). The same treatment method is reported in Wang, Yajun; Ke, Jiajun, Huagong Yejin, 9(4), 66-72, 1988.
The use of additives to render equipment surfaces resistant to scale adhesion in slurry heat exchangers conveying bauxite ore slurries is mentioned in Liu, Zigao; Huagong Yejin, 11(4), 326-32, 1990.
None of the art with which the applicant is familiar discloses the use of specific classes of polymeric organic nitrogenous compounds to modify the morphology of desilication products and thereby reduce their tendency to adhere to equipment surfaces. Nor is there art that indicates the stabilization of siliceous species in spent Bayer liquor by polyacrylamide, acrylamide/acrylic acid copolymers, polymeric quaternary ammonium compounds or polyamine compounds, or any other treatment type.